Organic electroluminescence means to convert electrical energy into light energy by using organic materials. That is, when voltage is applied between an anode and a cathode while aligning an organic material layer between the anode and the cathode, holes are injected into the organic material layer from the anode and electrons are injected into the organic material layer from the cathode. When the holes meet the electrons, excitons may be formed and such excitons generate light when they fall to the ground state.
Recently, research and studies are being actively carried out for fabricating displays or illumination units by using the electroluminescent phenomenon. In addition, techniques for depositing organic material layers in the form of a single layer or multi-layers are being actively studied in order to achieve effective organic light emitting devices. Most of currently available organic light emitting devices include an electrode layer and an organic material layer deposited in the form of a planar structure. Among those organic light emitting devices, an organic light emitting device having a planar multi-layer structure including an electrode layer and four organic material layers, such as a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer as shown in FIG. 1, has been widely used.
Referring to the organic light emitting device having a planar structure shown in FIG. 1, if an anode is a transparent anode and a substrate is a glass substrate, light generated from an organic material layer may pass through the transparent anode and the glass substrate. At this time, the light generated from a light emitting layer may travel through two different paths. Firstly, the light can be emitted out of the organic light emitting device. Secondly, the light may remain in the organic light emitting device while being reflected totally from the glass substrate or a surface of the anode. Among light generated from the light emitting layer, about ½n2 of the light can be emitted out of the organic light emitting device (wherein, n is a refractive index of the organic material layer). If the refractive index of the organic material layer of the device is 1.7, less than 17% of the light generated from the device can be emitted out of the organic light emitting device.
To solve the above problem and emit a large amount of light out of the organic light emitting device, an organic light emitting device structure including non-planar layer, i.e. non-planar structure has been suggested. The organic light emitting device having a structure a non-planar structure, can be fabricated through the following two methods.
According to a first method, a photonic crystal having a gravure pattern is formed on a glass substrate through a photolithography process before a transparent anode is deposited on the glass substrate (see, U.S. Publication No. 2003/0057417 and a document Appl. Phys. Lett. 82, 3779 issued on 2003 by Y. Lee et al.), or a corrugated pattern is formed on the glass substrate by using an interference of light(see, WO 2000/70691 and a document Adv. Mater. 13, 123 issued on 2001 by B. J. Matterson et al.), so as to improve light emitting efficiency. In detail, the former deposits an anode layer on the glass substrate, after forming the photonic crystal on the glass substrate and planarizing a surface of the glass substrate by using SiNx. The latter deposits an electrode layer and an organic material layer on the glass substrate while maintaining a corrugated pattern, after forming the corrugated pattern of transparent polymer on the glass substrate by using photoresist materials and an interference of light.
According to a second method, after fabricating an organic light emitting device having a planar structure as shown in FIG. 1, a micro-sized lens structure (see, WO 2003/007663 and a document J. Appl. Phys. 91, 3324 issued on 2000 by S. Moller et al.) or a millimeter-sized lens structure (see, WO 2001/33598) is attached to a surface of a glass substrate of the organic light emitting device, thereby improving light emitting efficiency of the device.
The above two methods can improve the light emitting efficiency of the light emitting device. However, the above two methods do not suggest a hemispherical recess structure formed on a substrate having high reflectance in order to effectively emit light. In addition, the above two methods represent problems when they are applied to an available light emitting device.
That is, the first method uses the photolithography process, so it is impossible to form the photonic crystal structure or the corrugated structure over a large-sized area. That is, in order to fabricate the light emitting device using the photonic crystal structure, it is necessary to sequentially carry out a deposition process, a photolithography process, and an etching process. At this time, the substrate must be processed at least two times under a vacuum state. In addition, in order to fabricate the light emitting device using the corrugated structure, it is necessary to perform the photolithography process by using an interference of light. However, the photolithography process is not adaptable for forming a uniform corrugated structure over a substrate having an area more than a few cm2.
The second method represents limitations when it is applied to display, because the lens structure has a size within a range of about a few micrometers to a few millimeters. In addition, the second method is not adaptable for a large-sized area due to preparation work thereof. According to the lens structure disclosed in WO 2003/007663, a minimum surface size of the lens structure is defined as a few μm such that minimum surface size of the lens structure must be larger than a maximum wavelength of visible ray emitted from the organic light emitting device. In addition, according to the lens structure disclosed in WO 2001/33598, the size of the lens structure must be larger than a size of one unit of an organic light emitting device.